The adhesive bonding of wire-shaped individual elements for manufacturing stacks of clips is a process known in the prior art. The diameters of the individual elements or individual wires of the stack of clips vary depending on the use and type of fixation by means of the clips. For extra-fine wire clips used in offices, a diameter of the individual wires is typically not more than 0.6 mm, whereas fine wire clips with a diameter of not more than 1 mm represent the commonest staples with the largest field of use. On the other hand, industrial clips generally have diameters of more than 1 mm, wherein wire clips with a diameter of more than 5 mm scarcely have an industrial significance any more.
Metal staples have by far the greatest technical importance and are generally manufactured by two techniques, viz. firstly by means of a wire drawing machine, through which a wire passes, the individual staples are cut off and their ends are sharpened, and then the individual staples are packed onto an essentially U-shaped rack. Solvent-containing adhesive paint is usually applied on the top and side surfaces of the individual adjacent staples on the rack of touching staples. Excess adhesive is wiped off by means of a knife or a wiper and the solvent driven off by heating the staples in an oven or for example by electrically heating the rack. In a second technique of the prior art, a number of wires, for example several hundred wires, are laid side by side to form a flat ribbon and the ribbon is coated underneath and/or on top with paint (“linear band process”). After drying, the ribbon of wires is rolled up and the rolled up ribbon of wires is then cut up and bent to form staple strips.
A solvent-based paint-like substance is usually used in both processes or in their modifications, wherein the viscosity and the solids content need to be carefully adjusted. This causes a simply limited manufacturing rate, because the solvent must first of all be completely removed from the adhesive coating by a time-intensive heat treatment. A manufacturing process of this type for stacks of clips with solvent-containing adhesive formulations is disclosed in DE 19545571.
A solvent-free process with UV curable adhesive bonding of individual wires arranged in parallel in a linear band process for manufacturing rolled up ribbons of wires is described in WO 94/14553.
DE 2622758 likewise teaches an improved process for manufacturing stacks of metallic clips in which an adhesive in the form of a radiation-curable, solvent-free, polymer mixture is applied onto a number of individual metal clips or wires arranged side by side. In particular, synthetic resins based on acrylate-modified polyurethanes, polyethers, polyesters and epoxides are suitable paint systems that cure under UV radiation in the presence of multifunctional crosslinking acrylates and monofunctional acrylate diluents. However, DE 2622758 teaches the application of a single layer system, such that the possibilities for regulating the integrity of the adhesively bonded clip stack remain limited. “Integrity” is understood by the person skilled in the art of producing clip stacks to mean the mechanical stability of the adhesively bonded stack, concomitant with sufficient flexibility of the composite for rolling up the ribbon of wires onto a roller in the linear band process.
Accordingly, the paint system has to be adapted such that the polymeric coating is neither too soft, which would result in a high attrition of the pressing jaws as the ribbon of wires is being shaped, thereby rapidly leading to a shutdown of the production process, nor too hard, which would cause the staples to be released too easily at their predetermined breakage point which can lead to claims from the end-user.